Unlike most cancers, leukemias occur at elevated rates in rural and farm populations. Previous studies have been unsuccessful in identifying any specific risk factors for leukemia in rural areas or farming. Using incidence data from the Iowa Cancer Registry, we have completed extensive preliminary ecologic-epidemiologic studies which have indicated there are elevated leukemia rates in Iowa which are strongly associated with: 1) male sex, 2) rural residence, 3) proximity to high cattle population densities, and 4) proximity to dairy herds with cases of bovine lymphosarcoma. The elevated rates and the associations noted were primarily associated with acute lymphoid leukemia (ALL). These preliminary data are suggestive that cattle and perhaps the bovine leukemia virus (BLV) may be factors in the high rates of leukemia seen, at least for ALL. We propose a case-control study to verify the ecologic relationships seen in the preliminary studies, and to investigate on an individual basis the role of various factors in the rural and agricultural environment that may contribute to the observed high leukemia rates. Approximately 200 cases of ALL (male and female) will be drawn from the Iowa Cancer Registry for the 66 rural counties. Two normal controls for each case will be selected from the same counties, and the cases and controls will be matched for age, sex, and county of residence. Personal interviews of each case and control will be completed to obtain data on: 1) environmental or occupational exposure to cattle, 2) environmental or occupational exposure to Bovine Leukemia Virus, 3) farm or non-farm occupation, and 4) various exposures within farming such as agricultural chemicals and other livestock species. Dairy herds of cases and controls will be tested serologically to determine the extent of BLV infection. The data will be analyzed to discern a possilbe cause-effect relationship of exposure to cattle, BLV, and other factors in the rural/agricultural environment.